In the framework of the EU project 'e-Ruption', a joint Italy-Portugal-Spain team conducted a seismicity study at the Fogo-Furnas volcanoes and adjacent areas on Sao Miguel Island, Azores. A sparse network of 14 stations equipped with both short-period and broad-band seismometers, and three dense arrays of short-period instruments recorded continuosly from April 1 to July 15, 2003. Aim of the experiment was to collect high-quality data in order to characterize the present-day level of activity of these volcanoes.
More than one thousand earthquakes were detected by our systems,and about 500 of these were energetic enough to allow for well-constrained hypocentral solutions. Most of this seismicity is associated to an intense swarm on April 26, 2003, when about 300 micro-earthquakes (Md < 2) occurred over a period of a few hours. This activity is concentrated in a small volume located NE of the summit Fogo crater, at depths ranging between 1 and 6 km b.s.l. As often observed in volcanic environments, the temporal pattern of energy release does not depict the typical mainshock-aftershock sequence. Precise, absolute location are obtained using a probabilistic inversion of arrival times. We apply PCA and hypocenter-collapsing techniques aimed at retrieving the most significant planes along which earthquakes occurred. The results are consistent with the focal mechanism obained from P-wave polarities, and indicate a complex pattern of faulting where fractures of different orientation and dip cluster in a region of a few square km. Waveform similarities and temporal evolution of the April 26 swarm suggest repeated rupture of the same patch of fault, therefore leading to the hypothesis that fluids circulation may have played a major role in the triggering processes of this and other similar seismic sequences. Vp/Vs ratios, determined through application of a modified Wadati technique, range over the 1.4-1.7 interval. These values are significantly lower than those usually observed in active volcanic areas, and may be attributed to the presence of boiling fluids associated to the geothermal system. A preliminary 1D velocity model to be used for a future tomographyc study is obtained from inversion of P- and S-waves travel times. A first image of lateral velocity heterogeneities is gained by analyzing travel-time residual for distant quakes and discrepancies among backazimuths at the array sites and hypocenter solutions obtained from inversion of arrival times.